1. Field of the Invention
This invention relates to heating system furnaces of the type used for heating buildings and homes, and more particularly, to a control system and method for ensuring safe operation of the furnace and for protecting the furnace in the event of a circulating air blower failure or severe air flow restriction.
2. Background of the Invention
Conventional forced air heating system furnaces, such as those commonly installed in homes, typically use a means for moving air, such as a circulating air blower, along an air flow path through a chamber embodying a heat exchanger. The circulating air blower, which is operated by a drive motor, forces air past the coils of the heat exchanger to heat the air, and then directs the heated air through outlet ducts into the space to be heated. Heat is supplied to the heat exchanger by combustion product gases entering the heat exchanger coils from a fuel burner controlled by a fuel valve. The relative positioning of the circulating air blower, drive motor and furnace heat exchanger can provide a categorization of these heating system furnaces as being "downflow" furnaces, wherein the circulating air blower and its drive motor are located above the heat exchange:; as "horizontal" furnaces, wherein the air blower and its drive motor are located to one side of the heat exchanger; or as "upflow" furnaces, wherein the air blower and the drive motor are located below the heat exchanger.
Unfortunately, in forced air heating system furnaces, circumstances can arise that will bring about overheating conditions in the enclosure housing of the furnace. If insufficient air is being delivered by the circulating air blower to the furnace chamber containing the heat exchanger, an overheated and unsafe condition can result while the fuel burner is actively combusting fuel. A similar condition will arise if the circulating air blower ceases to function properly or completely shuts down. This problem is particularly pronounced in the "horizontal" and "downflow" forced air furnaces because the heated air from the heat exchanger coils will heat the air surrounding the circulating air blower, the drive motor and the furnace filter. The resulting overheated condition arises as a consequence of the lack of air flow in the enclosure housing and the consequential inability to remove the heat generated from the burner.
Air flow through the furnace can become inadequate as the result of a variety of factors, such as a malfunctioning air blower or an obstruction of the air path, e.g., a clogged air filter or a blockage of either the input or output air ducts of the furnace. Because the possibility exists for a low or no air flow condition to occur in the furnace, various building codes have been implemented to provide safety standards to preclude an overheated condition of the furnace by stopping the flow of fuel to the burner to contain internal furnace temperatures within prescribed limits. These temperature limits are required not only to meet safety standards, but also to minimize damage to the air filter and/or other temperature sensitive parts of the furnace. An example of a test furnace designed to enforce such standards is detailed by ANSI standard Z21.47a, which is incorporated herein by reference in its entirety.
To maintain the prescribed safety temperature limits during a malfunction, it is known, as described by Z21.47a standard, to stop the flow of fuel to the burner during an overheating condition based on information provided by a temperature sensing switch. The temperature limit switch opens itself or switches to the OFF position to interrupt the flow of fuel to the furnace when the temperature of the air leaving the furnace exceeds a preset temperature of about 185.degree. F. and resets itself or switches to the ON position when the temperature of the air leaving the furnace falls below about 185.degree. F. This limit switch is called a "primary limit switch". The primary limit switch will self-reset to ON when the temperature drops below about 185.degree. F.
In addition, for a furnace for installation in the horizontal position or downflow position, in the event of a failed circulating air blower, a temperature limit switch must open to interrupt the flow of fuel to the furnace to prevent air temperatures at the inlet of the furnace (1) from exceeding 175.degree. F. above the ambient temperature prior to or during the first five cycles of the limit switch and (2) from exceeding 90.degree. F. above the ambient temperature on the sixth and subsequent cycles of the limit switch. A second temperature limit switch called an "auxiliary limit" may or may not be required to perform this function. The auxiliary limit switch may be of the manual reset type, typically reset by service personnel, after inspecting the furnace for and repairing any damage causing the loss of circulating airflow.
A fuel valve responsive to the state of the primary limit switch and the auxiliary limit switch stops fuel flow to the furnace when either of the switches are in the OFF condition. When both switches are ON, fuel flow is allowed to be restored to the fuel burner in the furnace. Fuel may then enter the furnace to be combusted in response to a demand signal from a thermostat external to the furnace. Conversely, when either switch is in the OFF position, indicating internal furnace temperatures in excess of the maximum allowable temperatures, fuel flow to the fuel burner is discontinued and signals from the thermostat for heat are ignored. In this manner, fuel will be prevented from entering the fuel burner when the internal furnace temperature rises above maximum allowable temperatures.
Requiring two switches for the operation of a heating control system in a forced air furnace, however, presents several disadvantages. For example, the operation of the second "lockout" switch may necessitate a service call whenever false alarms, not related to furnace operation, improperly trigger the switch and thereby preclude fuel flow. False alarms may arise due to power failures. The false activation of this switch adds considerably to the cost of ownership of the furnace because of the required service call.
Other disadvantages inherent in the presence of a second switch are the cost of the switch and its installation with associated circuitry, and the reduced reliability of the furnace. Reliability is reduced because the second switch increases the number of parts required for the operation of the furnace, thereby increasing the possibility of failure. In effect, the presence of the second "lockout" switch reduces overall reliability of the furnace.
Various attempts have been made to address the foregoing problems, including the employment of a microprocessor to control furnace operation in response to overtemperature conditions. For example, U.S. Pat. No. 4,951,870 issued to Ballard et al. on Aug. 28, 1990, discloses an apparatus and control method for responding to furnace overtemperature conditions wherein a microprocessor is used to count the number of occurrences of automatic resets encountered by an auxiliary limit switch that opens in response to overtemperature conditions resulting from a failure of the blower motor. When the number reaches a predetermined level, the furnace system is caused to lockout.
U.S. Pat. No. 4,502,625 issued to Mueller on Mar. 5, 1985 discloses a furnace control apparatus for a downflow forced air furnace having a microprocessor and thermostat to initiate and control the flow of gas to the burner for the start-up of the furnace. A fuel control means is provided for turning off the fuel supplied to the furnace when the temperature of the heating air remains below a predetermined reference temperature within a preset time following the start of the operation of the air circulator. The microprocessor is substituted for two temperature limit switches by a program for controlling the gas valve for stopping gas flow to the burner if the temperature of the air exiting the compartment of the heat exchanger exceeds a predetermined temperature stored in the microprocessor memory (in the event of a restriction to the air entering the furnace) or is below a predetermined temperature by a preset amount (in the event of a circulator failure).